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Examples with recursion: string-of-char-list?
The function string-of-char-list? is a predicate (a boolean function) that finds out if a string is formed exclusively by characters from a particular alphabet
(define (string-of-char-list? str char-list)
  (string-of-char-list-1? str char-list 0 (string-length str)))

(define (string-of-char-list-1? str char-list i lgt)
  (if (= i lgt)
      (and (memv (string-ref str i) char-list)
           (string-of-char-list-1? str char-list (+ i 1) lgt))))
Applications of string-of-char-list?.